In a typical DC-DC converter circuit 100 illustrated in FIG. 1, the converter circuit receives a voltage from a DC source 110, which may be a DC voltage supply that produces a DC voltage from an AC source (not shown). In the illustrated converter circuit in FIG. 1, the DC source is illustrated as a conventional battery, and the voltage from the DC source is provided on a VRAIL supply line 112. The voltage on the VRAIL supply line is referenced to an input ground reference 114. A first semiconductor switch (e.g., a power metal oxide semiconductor field effect transistor (MOSFET) or bipolar junction transistor (BJT) 120 has a first terminal connected to the VRAIL supply line and has a second terminal connected to a common switching node 122. A second semiconductor switch (MOSFET or BJT) 124 has a first terminal connected to the common switching node and has a second terminal connected to the input ground reference. Together, the two switches operate as a half-bridge circuit 126 to produce a switched DC voltage on the common switching node.
The control terminal (e.g., gate of a MOSFET or base of a BJT) of the first switch 120 is connected to a first output 132 of an integrated circuit controller 130. The control terminal of the second switch 124 is connected to a second output 134 of the controller. The controller operates in a conventional manner to turn on the first switch to couple the common switching node 122 to the VRAIL supply line 112; and then turn on the second switch to couple the common switching node to the input ground reference 114. When one of the switches is turned on, the other switch is turned off. The two switches are turned on and off at a selected repetition rate and with selected duty cycles to produce a voltage on the common switching node that alternates between the VRAIL voltage and ground.
The common switching node 122 of the half-bridge circuit 126 is connected to a resonant tank circuit 140 that includes a resonant circuit inductor 142, a first clamping diode 144, and a second clamping diode 146. A first terminal of the resonant circuit inductor 142 is connected to the common switching node 122 of the half-bridge circuit. A second terminal of the resonant circuit inductor is connected to the anode of the first clamping diode and to the cathode of the second clamping diode at a resonant tank node 148. The cathode of the first clamping diode 144 is connected to the VRAIL supply line 112. The anode of the second clamping diode is connected to the input ground reference 114. The two clamping diodes prevent the voltage on the resonant tank node from exceeding the VRAIL voltage by more than one diode forward voltage drop and from going below the input ground reference voltage by more than one diode forward voltage drop.
The resonant tank circuit 140 further includes a resonant circuit capacitor 150 and the primary winding 162 of an output transformer 160. A first terminal of the primary winding is connected to the resonant tank node 148. A second terminal of the primary winding is connected to a first terminal of the resonant circuit capacitor. A second terminal of the resonant circuit capacitor is connected to the input ground reference 114.
The output transformer 160 includes a center-tapped secondary winding 170 having a first winding half 172, a second winding half 174 and a center tap 176. The first winding half 172 is connected between the center tap and a first secondary output terminal 180. The second winding half 174 is connected between the center tap and a second secondary output terminal 182. The center tap 176 is connected to an output ground reference 184. The output ground reference is isolated from the input ground reference 114 by the output transformer. Accordingly, the output transformer may also be referred to as an isolation transformer.
The first secondary output terminal 180 of the output transformer 160 is connected to the anode of a first rectifier diode 190. The second secondary output terminal 182 is connected to the anode of a second rectifier diode 192. The cathodes of the two rectifier diodes are connected together at an output node 194. A filter capacitor 196 is connected between the output node and the output ground reference 184. A DC load (“LED LOAD”) 198 is connected across the filter capacitor between the output node and the output ground reference. In the illustrated embodiment, the DC load includes a plurality of light-emitting diodes (LEDs) connected in series or connected in a series-parallel combination.
In operation, the switched DC voltage on the common switching node 122 is AC-coupled to the primary winding 162 of the output transformer 160. Accordingly, an AC voltage is produced on the secondary winding 170 of the output transformer. The AC output of the secondary winding is rectified by the two rectifier diodes 190, 192 to produce a DC voltage (VLED) across the filter capacitor 196 to drive the LEDs of the DC load 198.
In the conventional resonant tank circuit 140 illustrated in FIG. 1, the resonant circuit inductor 142 and the output transformer 160 are two entirely separate magnetic components. For example, FIG. 2 illustrates a conventional resonant inductor 142. FIG. 3 illustrates an exploded view of the resonant inductor. As illustrated, the resonant inductor includes a bobbin 200 having a coil 202 wound around a central passage 204. A first E-core 210 of the inductor has a center leg 212 inserted into the central passage from a first end of the bobbin. The first E-core has a first outer leg 214 and a second outer leg 216 positioned on opposed sides of the bobbin. A second E-core 220 of the inductor has a center leg 222 inserted into the central passage from a first end of the bobbin. The second E-core has a first outer leg 224 and a second outer leg 226 positioned on opposed sides of the bobbin. The bobbin further includes a first pin rail 230 and a second pin rail 232 at opposed ends of the bobbin. Each pin rail supports a plurality of pins 234. The ends of the winding (not shown) of the coil are connected to selected pins on one or both of the pin rails. For example, in a conventional inductor having a single coil winding, a first end of the winding is connected to a pin on the first pin rail and a second end of the winding is connected to a pin on the second pin rail. Alternatively, both ends of the winding can be connected to respective pins on the same pin rail.
FIG. 4 illustrates a conventional output transformer 160. FIG. 5 illustrates an exploded view of the output transformer. As illustrated, the transformer includes a bobbin 240 having a coil 242 wound around a central passage 244. A first E-core 250 of the transformer has a center leg 252 inserted into the central passage from a first end of the bobbin. The first E-core has a first outer leg 254 and a second outer leg 256 positioned on opposed sides of the bobbin. A second E-core 260 of the transformer has a center leg 262 inserted into the central passage from a first end of the bobbin. The second E-core has a first outer leg 264 and a second outer leg 266 positioned on opposed sides of the bobbin. The bobbin further includes a first pin rail 270 and a second pin rail 272 at opposed ends of the bobbin. Each pin rail supports a plurality of pins 274. The ends of the windings (not shown) of the coil are connected to selected pins on one or both of the pin rails. For example, in a conventional transformer, the two ends of the primary winding may be connected to respective pins on the first pin rail, and the two end terminals and the center tap of the secondary winding may be connected to three pins on the second pin rail.
As shown in FIGS. 2-5, each of the inductor 140 and the output transformer 160 occupies a respective surface area defined by the spacing between the respective first and second pin rails, the widths of the pin rails and spacing required between adjacent components. For example, FIG. 6 illustrates a first plan view of the inductor and transformer positioned longitudinally with respect to each other on a typical printed circuit board with a minimal spacing between the two components. FIG. 7 illustrates the two components positioned laterally with respect to each other. In either configuration, the surface area occupied by the two components is considerably greater than the surface area occupied by the transformer alone or by the inductor alone.